The present invention relates to a system for transmitting video signals.
As one of bi-directional video signal transmission systems is known a split type bi-directional CATV system which is equipped with coaxial cables having a star network topology and which has fixed frequency range for transmitting and reaceiving each. The video signal is modulated to the frequency in the fixed range for transmitting and multiplexing to RF upstream, and it is block-converted to the fixed range for receiving at the center and multiplexed to RF downstream. Thus, the split type bi-directional CATV system requires two fixed channels for each video source. In a typical arrangement for additional use as CATV, the number of the channels for transmitting video signals becomes on the order of 3 because of sufficiently ensuring the CATV channels. There are problems which arise with such an arrangement, however, in that, a channel allocated for one video source cannot be used for other video source, that is, a channel for transmitting video signal cannot be shared by plural video sources. Even if taking the high-split system, the number of effective video sources is half the number of channels in RF signal band, so it implies that the RF signal band cannot be utilized effectively. In addition, difficulty is encountered to prevent the wire tapping by a third party because it is easy to retrieve video signals by setting the channel of the tuner. This can provide a problem in security in one-to-one image communication. Moreover, the provision cost increases because of using a bi-directional amplifier as the repeating amplifier.